1. Field of the Invention
This invention relates to single and multiband oscillators, particularly high-Q precision integrated oscillators.
2. Description of the Related Art
An oscillator is an essential part of nearly every electronic instrument including signal, function and pulse generators, digital multimeters, oscilloscopes, computers and RF transmitters and receivers. At high frequencies, the preferred technique for generating the local oscillator signal is an LC-controlled oscillator, in which a tuned LC network is connected in an amplifier-like circuit to provide gain at its resonant frequency .omega..sub.0 =1/LC. Overall positive feedback is then used to cause a sustained oscillation to build up at the LC's resonant frequency.
The performance of many RF oscillators depends critically on how precisely specific inductance and capacitance values can be provided in a circuit, and on the quality factor or "Q" of the circuit's reactive components. Q is defined as the maximum amount of energy stored divided by the amount of energy lost during one complete cycle. Thus, the circuit's frequency response peaks at a frequency determined by the circuit's inductance and capacitance values, and the width of the peak depends on the Q value of the circuit's components. Unwanted resistance tends to lower Q and flatten the frequency response.
Obtaining high-Q components with precise inductance values has traditionally been accomplished by either hand-selecting a component having desired characteristics from a batch, or by trimming the component as needed after manufacture. However, even state-of-the-art laser trimming methods impose limits on how closely one can get to a desired value, and both hand-selecting and trimming are expensive and labor-intensive. Furthermore, the lack of integration associated with discrete components increases both the size and cost of the oscillator to the point that such a system is not feasible in current microelectronic applications.
Integration enables the oscillator's inductors and capacitors to be made simultaneously with other circuit components, reduces the distance between a circuit's reactive and other components, eliminates the need for parasitic capacitance-causing wire bonds, and reduces the circuit's space and weight requirements, which are typically at a premium in wireless communications devices. However, integrated reactive components are difficult to trim to specific values and require a considerable amount of die area.
One method of providing a precise inductance (capacitance) value requires a number of fixed inductors (capacitors) to be fabricated on a substrate, which are then selectably interconnected with solid-state or off-chip switches to produce a desired value. Integrated switches capable of handling microwave frequencies are typically implemented with gallium arsenide (GaAs) MESFETs or PIN diode circuits. At signal frequencies above about 900 MHZ, such as those used by a cellular phone, these switching devices or circuits typically exhibit an insertion loss in the `ON` (closed) state of about 1 db and a less-than-infinite isolation, typically no better than -30 dB in the `OFF` (open) state. The insertion loss severely lowers Q, which causes the frequency response of the circuit to flatten out, lowering its selectivity and widening its bandwidth, often rendering the circuit impractical for use in wireless communications devices.
Consequently, trimmable RF oscillators that operate at frequencies above approximately 1 GHz use multiple chip sets that are fabricated with different technologies to optimize the capacitors, the FET switches, and the control circuitry, respectively, to get satisfactory insertion loss and isolation. For example, the capacitors may be formed on a glass substrate, the FET switches on a gallium arsenide substrate, and the control circuitry on a silicon substrate.
Some systems such as signal generators and RF transmitters/receivers require multiple frequency sources with each source providing a precise narrowband signal. Typically, these systems use a number of separate oscillators that are individually designed and trimmed to their respective resonant frequencies. This approach provides high quality and uniform precision and narrow bandwidth. However, the cost of multiple oscillators that are each dedicated to a single frequency can be significant.
Another known approach is to use solid state varactor diodes to provide a tunable capacitor. The varactor's capacitance is set by a bias voltage, which is generated by a sub-circuit that can consume a significant amount of when tuning the varactor. A low power-consuming varactor's tuning ratio is limited to about 4:1, which limits its usefulness for some wideband applications such as the frequency agile secured communications. Furthermore, the signal current applied to the varactor will affect the capacitance, inducing some measure of error.
Darrin J. Young and Bernhard E. Boser, "A Micromachined Variable Capacitor for Monolithic Low-Noise VCOS," Technical Digest of the 1996 Solid-State Sensor and Actuator Workshop, Hilton Head, S.C. pp. 86-89, 1996 discloses an aluminum micromachined variable capacitor for use as the tuning element in a voltage-controlled oscillator (VCO). This device is fabricated on top of a silicon wafer using conventional deposition techniques, and consists of a thin sheet of aluminum suspended in air above the substrate and anchored with four mechanical folded-beam suspensions acting as springs to form a parallel-plate capacitor. Compared to varactor diodes, this approach is amenable to monolithic integration in a standard electronic circuit process without sacrificing performance.
However, Young's parallel-plate structure has a number of drawbacks. First, the parallel-plate structure has a maximum vertical deflection of 1/3 of the initial gap between the parallel plates, which corresponds to a limited tuning range of at most 50%. Second, the capacitor's Q is 62 at 1 GHz, limited by the device parasitics and the amount of metal that can be deposited. Third, to isolate the DC control circuit from the signal voltage a pair of large inductors must be connected between the control circuit and the capacitor. On-chip inductors are of generally poor quality and discrete inductors reduce the overall circuit integration. Lastly, since the capacitance value is not fixed, it is sensitive to fluctuations in the signal voltage and external vibrations.